The document “Transverse Mode Selection in Large-Area Oxide-Confined Vertical-Cavity Surface-Emitting Lasers Using a Shallow Surface Relief” (H. Martinsson, J. A. Vukusic, M. Grabherr, M. Michalzik, R. Jäger, K. J. Ebeling, A. Larsson; IEEE Photonics Technology Letters, Vol. 11, No. 12, December 1999, pages 1536–1538) describes a method for producing a VCSEL laser, in which a so-called “semiconductor relief” is produced on the surface of the VCSEL laser. The function of the semiconductor relief is to suppress higher modes of the light generated in the active zone of the laser and to leave only the fundamental mode of the light as far as possible uninfluenced. In essence, the functioning of the semiconductor relief is based on the fact that higher modes have a field distribution in the case of which the light is guided principally at the edge of the radiation lobe of the light. In contrast thereto, the fundamental mode has a radiation behavior in the case of which the light is situated principally in the inner region of the radiation lobe of the light. The semiconductor relief thus preferably suppresses higher modes, so that principally or exclusively the fundamental mode of the VCSEL laser is coupled into optical components arranged downstream of the VCSEL laser.
In the case of the VCSEL laser in accordance with the already cited document “Transverse Mode Selection in Large-Area Oxide-Confined Vertical-Cavity Surface-Emitting Lasers Using a Shallow Surface Relief”, the semiconductor relief is arranged on the top side of the VCSEL laser, that is to say above the upper mirror or mirror stack of the VCSEL laser. The semiconductor relief is thus separated from a current aperture of the VCSEL laser by the upper mirror layer of the laser. In the case of the VCSEL laser shown in the document, the current aperture has an area size with a diameter of 15.5 μm. In this case, the area size of the semiconductor relief is understood to be the area size of the inner raised region of the semiconductor relief. The current aperture arranged below the upper mirror layer of the VCSEL laser has an area size (or a diameter) which is larger than the area size (or the diameter) of the semiconductor relief. In concrete terms, the diameter of the current aperture is 20 μm.
The document “Increased-area oxidized single-fundamental mode VCSEL with self-aligned shallow etched surface relief” (H. J. Unold, M. Grabherr, F. Eberhard, F. Mederer, R. Jäger, M. Riedl, K. J. Ebeling; Electronics Letters, 5th Aug. 1999, Vol. 35, No. 16) furthermore discloses a method for producing a laser, in which the semiconductor relief and the current aperture are produced in a self-aligning manner. This means that the current aperture is arranged in a concentrically aligned manner relative to the semiconductor relief. The self-alignment is achieved by virtue of the fact that both the position of the semiconductor relief and the position of a mesa structure of the VCSEL laser are defined in the same mask step. The mesa structure is produced in subsequent etching steps, the semiconductor relief inevitably remaining arranged centrally in the mesa structure. The area size of the current aperture is defined in the context of an oxidation step during which the sidewalls of the etched mesa structure are oxidized. This oxidation step effects lateral “oxidation into” the current aperture layer contained in the mesa structure. The semiconductor relief is separated from the current aperture by an upper mirror layer.
Accordingly, the invention provides a laser production method in which a current aperture and a semiconductor relief are produced; the area size of the semiconductor relief and the area size of the current aperture are defined in the same production step.
An essential advantage of the method according to the invention is that it is always ensured that the area size of the semiconductor relief and the area size of the current aperture are in a fixed relationship with respect to one another, since both the semiconductor relief and the current aperture are defined in the same production step. By way of example, if production tolerances occur on account of fluctuations in the production conditions (e.g. production temperature, moisture fluctuations), then the area size of the semiconductor relief will change under certain circumstances; however, since the semiconductor relief and the current aperture are defined in the same production step, the area size of the current aperture will also simultaneously be affected by the fluctuations in the production conditions, so that its size changes as well. As a result, the area size of the semiconductor relief and the area size of the current aperture will consequently change relatively “similarly”, so that they will nevertheless have a size ratio with respect to one another that corresponds to the actually desired size ratio without production tolerances. The area size of the semiconductor relief and the area size of the current aperture thus comply with a predetermined size ratio “in a self-scaling manner”; a “self-scaling” does not occur in the case of the previously known method described in the introduction, because the definition of the semiconductor relief and the definition of the current aperture are effected in separate production steps.
A further essential advantage of the common production process for the semiconductor relief and the current aperture is that the yield in the production of the lasers is increased: it is because generally each current aperture diameter is matched only by a specific semiconductor relief diameter in order to obtain a single-mode radiation with a maximum optical output power at the output of the laser. The fixed scaling of the semiconductor relief and of the current aperture considerably increases the production yield and the process stability.
The area size of the semiconductor relief and the area size of the current aperture may be defined for example in an oxidation step; this procedure is advantageous in particular because current apertures are usually produced in an oxidation step. Consequently, in this refinement of the method, the size of the semiconductor relief is defined during the oxidation of the current aperture.
During the production of the VCSEL laser, an oxidizable auxiliary layer for the definition of the area size of the semiconductor relief and an oxidizable current aperture layer for the definition of the current aperture are preferably subjected to the common oxidation step. In this case, the ratio between the oxidation rate of the oxidizable auxiliary layer and the oxidation rate of the current aperture layer defines the size ratio between the area size of the resulting semiconductor relief and the area size of the resulting current aperture. The oxidizable auxiliary layer and the current aperture layer may be for example layers made of AlGaAs material, the proportion of aluminum determining the oxidation rate: the higher the proportion of aluminum, the greater the oxidation rate.
During the production of the VCSEL laser, a mesa structure is preferably produced, which mesa structure encompasses or includes the oxidizable auxiliary layer and also the current aperture layer. The sidewalls of the mesa structure are subsequently oxidized, thereby also effecting oxidation “into” the oxidizable auxiliary layer and the current aperture layer within the mesa structure. The size ratio between the area size of the semiconductor relief and the area size of the current aperture is defined in this case.
In order to produce the VCSEL laser, it is possible, by way of example, firstly to arrange at least one semiconductor intermediate layer on the oxidizable current aperture layer of the VCSEL laser. The oxidizable auxiliary layer is subsequently arranged on the semiconductor intermediate layer. A covering layer is applied to the oxidizable auxiliary layer, for example by being grown epitaxially. The mesa structure is subsequently etched into the resulting layer stack, and the sidewalls of the mesa structure are subjected to the oxidation step. The oxidizable current aperture layer and the oxidizable auxiliary layer are laterally oxidized simultaneously during this oxidation step.
The VCSEL laser is completed particularly simply and thus advantageously by subsequently removing the oxidizable auxiliary layer in its oxidized regions, a region of the semiconductor intermediate layer being uncovered. The semiconductor intermediate layer is subsequently etched in the uncovered region down to a depth corresponding to the depth of the semiconductor relief to be produced. In addition, the covering layer and the non-oxidized regions of the oxidizable auxiliary layer are completely removed, thereby uncovering the semiconductor relief in the semiconductor intermediate layer. This then concludes the formation of the semiconductor relief.
Afterward, a mirror layer or a mirror layer stack comprising a plurality of mirror layers, which forms the upper mirror layer of the laser, is preferably deposited on the semiconductor relief. The semiconductor relief is thus arranged between the mirror layer of the VCSEL laser and the current aperture of the VCSEL laser.
In contrast to the previously known methods mentioned in the introduction, the area size of the semiconductor relief is preferably made to be larger than the area size of the current aperture in order to achieve an optimum radiation behavior.
The upper mirror layer (or the upper mirror layers) deposited on the semiconductor relief may be for example layer stacks or layer pairs made of dielectric materials, preferably made of aluminum oxide and titanium oxide.
Furthermore, it is regarded as advantageous if an upper electrical contact of the VCSEL laser is arranged in a self-aligned manner relative to the current aperture and relative to the semiconductor relief, as a result of which a homogeneous current injection is achieved.
The upper electrical contact is preferably an intra-cavity contact, that is to say a contact which makes contact with a semiconductor layer of the VCSEL laser that is arranged below the upper mirror layer of the VCSEL laser.
The intra-cavity contact may be formed for example on the semiconductor intermediate layer already mentioned above.
The invention furthermore relates to a vertically emitting laser, in particular a VCSEL laser, with an as far as possible optimum radiation behavior.
The invention provides a laser, in particular a VCSEL laser, with a semiconductor relief for radiating undesirable modes, in which the semiconductor relief is arranged between an upper mirror layer of the laser and a current aperture of the laser.
Disturbing higher modes of the laser can be suppressed particularly simply and thus advantageously if the area size of the semiconductor relief is chosen to be larger than the area size of the current aperture. One advantage of the area ratio chosen in this way between the semiconductor relief and the current aperture consists in avoiding incomplete depletion of charge carriers below the semiconductor relief and—caused by a slow diffusion process—impairment of the modulation behavior of the laser.
The mirror layer of the VCSEL laser preferably comprises layer stacks or layer pairs made of dielectric materials, preferably made of aluminum oxide and titanium oxide.
As already mentioned in the introduction the VCSEL laser may have an intra-cavity contact as the upper electrical contact; as an alternative or in addition, the second or “lower” electrical contact of the laser may also be an intra-cavity contact.